Currently, glass plates are used as most of substrates for organic EL or liquid crystal displays. However, the glass plates have a high specific gravity and the properties of being heavy, breakable, and rigid and are therefore considered to be difficult to adapt to the future weight reduction, flexibility, etc., of various displays. Accordingly, studies using plastic films such as polyimide or polycarbonate have been made as substitutes for the glass plates in recent years.
However, these plastic films, as compared with glass, have a high linear expansion coefficient and cause the warpage of the films or the fracture of electronic elements during the formation of the electronic elements (inorganic material) which requires a high-temperature process. Thus, the plastic films are difficult to put into practical use. Accordingly, for these purposes, there has been a demand for a novel material that has transparency and a low linear expansion coefficient comparable to those of glass and exhibits heat resistance and yellowing resistance against various process temperatures.
In recent years, cellulose ultrafine fiber having a fiber diameter of 1 μm or smaller has received attention. The cellulose ultrafine fiber is an assembly of cellulose crystals having a high modulus of elasticity and a low thermal expansion rate. A nonwoven fabric prepared from this ultrafine fiber and its composite with a transparent resin have been reported to have high transparency, a high modulus of elasticity, a low linear expansion coefficient, and flexible properties (Patent Literatures 1 and 2).
A method which involves introducing a substituent having electrostatic and/or steric functionality to a fiber raw material in order to facilitate nanofibrillation (defibration) of the fiber raw material is known as a method for producing cellulose ultrafine fiber having a small fiber diameter (e.g., Patent Literatures 3 to 6). The substituent having electrostatic and/or steric functionality, which has been introduced in ultrafine fiber, improves the dispersibility of the ultrafine fiber through electrostatic repulsion or the like. A sheet prepared using this ultrafine fiber has high transparency as compared with a substituent-unintroduced one.
Meanwhile, Patent Literature 7 discusses not only facilitating the nanofibrillation (defibration) of a fiber raw material but achieving the favorable water filterability and dewaterability of ultrafine fiber-containing slurry after nanofibrillation (defibration) while ameliorating time-dependent yellowing or thermal yellowing of the obtained ultrafine fiber and a sheet containing the ultrafine fiber. In this literature, improved ultrafine fiber is obtained by eliminating the introduced substituent from a slurry state after nanofibrillation (defibration) of the substituent-introduced ultrafine fiber.
Also, the heat treatment of the cellulose ultrafine fiber has been practiced. For example, studies have been made on a technique of suppressing staining and achieving high transparency, unstainability, a low linear expansion coefficient, and a high modulus of elasticity as a cellulose fiber composite by using cellulose heat-treated in the presence of a solvent after nanofiber formation (Patent Literature 8). Furthermore, a resin molded article in a sheet form having an improved tensile modulus of elasticity and a high fracture elongation is obtained by thermally denaturing cellulose oxide fiber for enhancement in its dispersibility in a resin and dispersing the fiber in the resin (Patent Literature 9).